In a typical cellular radio system or telecommunication system, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some telecommunication systems may also be called, for example, a “NodeB” in UMTS or “eNodeB” (eNB) in LTE. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole telecommunication system is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a Radio Network Controller (RNC) or a Base Station Controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunication System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunication suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) are distributed between the radio base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS system has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
FIG. 1 depicts the architecture of the LTE system. The E-UTRAN is made up of eNBs, which are connected to each other via the X2 interface. Both the S1 and the X2 interface may be divided into control plane (dashed lines) and user plane (solid lines) parts. eNBs are connected to a Mobility Management entity (MME) or a Serving Gateway (S-GW).
Handover is one of the important aspects of any mobile/telecommunication system when a user equipment moves from one cell to another. The telecommunication system tries to assure service continuity of the User Equipment (UE) by transferring the connection from one cell to another depending on several factors such as signal strength, load conditions, service requirements, etc. The provision of efficient handovers, minimum number of unnecessary handovers, minimum number of handover failures, minimum handover delay, etc., would affect not only the Quality of Service (QoS) of the end user equipment but also the overall telecommunication system capacity and performance.
In LTE, UE-assisted, network controlled handover is utilized. The network, such as the radio base station or a core network node, configures the user equipment to send measurement reports and based on these reports the user equipment is moved, if required and possible, to the most appropriate cell that will assure service continuity and quality. Handover is performed via the X2 connection, whenever available, and if not, it is performed via the S1 connection, i.e. involving the Core Network (CN). The X2 Handover process is shown in FIG. 2. The handover procedure may be sub divided into three stages of preparation or initiation, execution and completion. The handover is performed from a source eNB to a target eNB that are under control of a MME and connected to a serving Gateway. The handover procedure comprises different steps such as area restriction provided, Measurement control, UL allocation, Measurement reports transmitted, and a Handover (HO) decision is taken. The source eNB configures the UE measurement procedures according to the area restriction information. UE sends MEASUREMENT REPORT by the rules set by i.e. system information, specification etc. Source eNB makes decision based on MEASUREMENT REPORT and Radio Resource Management (RRM) information to hand off UE and issues a HANDOVER REQUEST message to the target eNB passing necessary information to prepare the HO at the target side. Admission Control may be performed by the target eNB. Target eNB prepares HO with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB. The HANDOVER REQUEST ACKNOWLEDGE message includes a transparent container to be sent to the UE as a Radio Resource Control (RRC) message to perform the handover. The UE receives DL allocation and the RRCConnectionReconfiguration message with necessary parameters and is commanded by the source eNB to perform the HO. The source eNB sends the Sequence Number (SN) STATUS TRANSFER message to the target eNB. After receiving the RRCConnectionReconfiguration message including the mobilityControllnformation, UE performs synchronisation to target eNB and accesses the target cell. The target eNB responds with UL allocation and timing advance. UE sends the RRCConnectionReconfigurationComplete message to confirm the handover to the target eNB to indicate that the handover procedure is completed for the UE. The target eNB can now begin sending data to the UE. The target eNB sends a PATH SWITCH REQUEST message to MME to inform that the UE has changed cell. The MME sends a Modify Bearer request message to the Serving Gateway. The Serving Gateway switches the downlink data path to the target side. The Serving gateway sends one or more “end marker” packets on the old path to the source eNB and then can release resources towards the source eNB. Serving Gateway sends an Modify Bearer Response message to MME. The MME confirms the PATH SWITCH message with the PATH SWITCH ACKNOWLEDGE message. The target eNB informs success of HO to source eNB by sending UE CONTEXT RELEASE and triggers the release of resources by the source eNB. The target eNB sends this message after the PATH SWITCH ACKNOWLEDGE message is received from the MME.
During the preparation or initiation stage, based on the measurement results, a source eNB receives from the user equipment, the source eNB decides whether to handover the connection to another eNB, called a target eNB, or not. If the decision is to handover, the source eNB sends a HANDOVER REQUEST message to the target eNB. The source eNB must indicate the cause of the HO in this message, which may be, e.g.                Handover Desirable for Radio Reasons,        Resource Optimisation Handover, and/or        Reduce Load in Serving Cell        
Thus the target eNB knows that the HO is due to resource optimization or to reduce the load in the serving cell.
If the target eNB is able to admit the user equipment, a message is sent to the user equipment to initiate the handover, and the handover execution state is entered. DL data arriving at the source eNB for the user equipment are then forwarded to the new target eNB.
The handover completion stage is entered once the target eNB and the user equipment are synchronized and a handover confirm message, see step 11 of FIG. 2, is received by the target eNB. After a proper setup of the connection with the target eNB is performed, which include the switching of the DL path in the serving gateway, the old connection is released and any remaining data in the source eNB that is destined for the user equipment is forwarded to the target eNB. Then normal packet flow may ensue through the target eNB.
The handover is triggered by a UE measurement report configuration. The UE measurement report configuration comprises the reporting criteria, whether it is periodic or event triggered, as well as the measurement information that the user equipment has to report.
The following event-triggered criteria are specified for intra-Radio Access Technology (RAT) measurement reporting in LTE:                Event A1: Serving cell becomes better than absolute threshold, better here meaning signal stronger than a threshold.        Event A2: Serving cell becomes worse than absolute threshold, worse meaning signal is weaker than a threshold.        Event A3: Neighbour cell becomes better than an offset relative to the serving cell.        Event A4: Neighbour cell becomes better than absolute threshold.        Event A5: Serving cell becomes worse than one absolute threshold and neighbour cell becomes better than another absolute threshold.        
The most important measurement report triggering event related to handover is A3, and its usage is illustrated in FIG. 3. Signal strength is defined along a vertical axis and time defined along a horizontal axis. The triggering conditions for event A3 may be formulated as:N>S+HOM  (eq. 1)
where N is the signal strength from a neighbouring cell and S is the signal strength of the serving cell, respectively, and HOM is the Handover Margin. HOM is the difference between the radio quality of the serving cell and the radio quality needed before attempting a handover. The radio quality is measured either using Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ).
The user equipment triggers an intra-frequency handover procedure by sending event A3 report to the eNB. Intra herein meaning that within the same frequencies and inter means between different frequencies. This event A3 occurs when the user equipment measures that the target cell is better than the serving cell with a margin “HOM”. The user equipment is configured over Radio Resource Control (RRC) when entering a cell, which RRC belongs to the UMTS WCDMA protocol stack and handles the control plane signalling of Layer 3 between the UEs and the eNBs, and the HOM may be calculated from the following configurable parameters:HOM=Ofs+Ocs+Off−Ofn−Ocn+Hys  (eq. 2)
where:                Ofs is a frequency specific offset of the serving cell        Ocs is a Cell Specific Offset (CIO) of the serving cell        Off is an A3-Offset        Ofn is a frequency specific offset of the neighbor cell        Ocn is the CIO of the neighbor cell        Hys is an hysteresis        
If the condition in the equation eq. 1 is satisfied and it remains valid or fulfilled for a certain period of time known as Time To Trigger (TTT) e.g. between point A and B, the user equipment sends a measurement report to the serving eNB see FIG. 3. In FIG. 3, event A3 is satisfied at point A and measurement report is sent at point B in time. When it gets the measurement report, the serving eNB initiates a handover towards the neighbour cell.
As discussed above, handover in LTE is controlled via several parameters. Incorrect parameter settings may lead to several problems such as Radio Link Failure (RLF), Handover Failure (HOF) and Ping-pong Handover, also known as Handover Oscillation, which is when the user equipment is handover back and forth within a short period of time.
RLF occurs if the handover parameters are set in such a way that the user equipment doesn't report handover measurements on time, the user equipment might lose the connection with the original cell before handover is initiated. For example, when the user equipment receives a certain number of N310 consecutive “out of sync” indications from lower layer, it assumes a physical layer problem is ensuing, and a timer T310 is started. If the user equipment doesn't receive a certain number of N311 consecutive “in sync” indications from the lower layer before the timer T310 expires, RLF is detected. RLF is also detected when random access problem is indicated from Medium
Access Control (MAC) layer or upon indication that the maximum number of RLC retransmissions has been reached.
HOF occurs if the connection with the original cell is lost while HO is on-going with the target cell. When the user equipment receives a Handover (HO) command, i.e. RRCConnectionReconfigurationRequest with mobilityControlInfo, as shown in FIG. 2, it starts a timer T304, and if this timer expires before the HO is completed, i.e. RRCConnectionReconfigurationComplete message is sent by the user equipment, a HOF is detected.
Improper handover parameter setting may make the user equipment handover back and forth between two neighbouring cells so called Ping-pong Handover or Handover oscillation. An example of this is a setting that makes the triggering conditions for the handover events A3 valid between the source and neighbour cells at the same time. FIG. 4 illustrates handover oscillation. A user equipment is said to have experienced handover oscillation if it stays in a target cell for a duration T that is less than a handover oscillation threshold Tocs, before it is handed back to the source cell. The oscillation rate may be defined as the ratio between the number of oscillations and the total number of HOs.
There is an upper boundary for an acceptable oscillation rate originating from e.g., core network load. Also the oscillation rate is related to end-user equipment performance.
On one hand oscillation are harmful as this induces additional signalling and delays, and on the other hand, oscillations allow the user equipment to be connected to the ‘best’ cell with best received signal strength. This needs to be balanced in order for the (end-) user equipment to experience the best performance.
When a RLF or HOF is detected by the user equipment, the user equipment starts a timer T311 and tries to re-establish the connection to the best available cell, e.g. the source cell, another cell belonging to the same source eNB or a neighbour cell belonging to another eNB. This is known as RRC Connection Reestablishment, and is shown in FIG. 5.
The user equipment includes the following information in the RRCConnectionReestablishmentrequest to the E-UTRAN:                Physical Cell ID (PCI) of the last cell the user equipment was connected to before RLF.        UE Identity: The Cell Radio Network Temporary Identifier (CRNTI) as well as        
MAC ID for context lookup, using which the last serving cell may identify the user equipment.                Re-establishment cause: Whether the request is due to handover failure, reconfiguration failure, or other causes.        
If the UE context is found in the cell, if it is the source cell or if it was a cell prepared for handover, i.e. handover was on-going when the RLF happened and the cell where the user equipment re-appeared already has the UE context, which was communicated to it from the source cell during Handover Request message exchange, the connection is re-established, sending a RRCConnectionReestablishmentcomplete. Otherwise, if the UE context is not available, or re-establishment didn't succeed before the timer T311 expires, the user equipment has to go to IDLE mode and have to tear down all the active bearers, if any, and may restart the bearer setups, if required.
Configuring all the HO parameters manually in order to avoid the aforementioned problems is too expensive and may be very challenging. As such, Mobility Robustness Optimization (MRO) has been introduced in the 3GPP standard specification to automate the dynamic configuration of handover parameters.
MRO tries to identify the following three situations, and based on the statistical occurrence of these, tries to adjust the HO parameters.                Too Late HO: A user equipment is handed over late to the target cell, so that the link to the source cell breaks before completing the handover.        Too Early HO: A user equipment is handed over to a candidate cell too early resulting in a radio link or handover failure in the target cell. The user equipment returns soon to the source cell via re-establishment procedures.        Handover to wrong cell: A user equipment is handed over to one target cell but it experiences a RLF within a short duration after that in the target cell and the user equipment re-establishes the connection at another cell. A proper parameter setting would have most probably have led to the handing over of the user equipment to the last target cell to begin with.        
MRO tries to gather statistics on the occurrence of Too Late HOs, Too Early HOs and HO to the wrong cell, and these statistics are used to adjust the handover parameters. One or more of the following handover parameters controlling the event driven reporting of the user equipment may be adjusted by MRO:                A threshold indicating how much stronger a certain candidate cell needs to be before it is reported to the serving cell.        A filter coefficient applied to the measurement before evaluation triggers are considered.        A time to trigger meaning the time window within which the triggering condition needs to be continuously met in order to trigger the reporting event in the user equipment.        
For example, a higher ‘too early handover’ ratio than desired may be counter-acted by increasing the threshold, delaying the triggering of A3 event. Another example may be the resolving of a higher ‘handover to wrong cell’ ratio than desired by increasing the threshold towards the first, unwanted, target cell.
Three main messages, namely RLF report, between the user equipment and eNBs, RLF INDICATION, between eNBs, and HANDOVER REPORT, between eNBs, are used by MRO to communicate and/or gather information regarding Too Early Handover, Too Late Handover and Handover to the wrong cell.
The eNB to which the user equipment is reconnecting to, either through a successful RRC re-establishment or via RRCConnectionSetup after IDLE mode, may ask for more detailed information about the failure after the connection is completed. This is done via the UE Information Request procedure, where the eNB may ask for RLF report, as shown in FIG. 6.
The user equipment responds by sending a UElnformationResponse message with a detailed RLF report which may include information such as:                A measurement result of the last served cell before RLF.        A measurement result of the neighbour cells performed before RLF.        A location info, which may include last co-ordinates as well as velocity of the UE when RLF was detected.        An enhanced CGI (E-CGI), and if that is not available Physical Cell ID (PCI), of the cell where RLF occurred.        An E-CGI of the cell that the re-establishment attempt was made at.        Whether the RLF occurred after the reception of a HO command, i.e.RRCConnectionReconfiguration message including the mobilityControllnfo.        The E-CGI where this message was received.        The elapsed time since the reception of this message.        The RLF type: i.e. whether it is a normal radio link failure or a handover failure.        
Using the information disclosed above the eNB may deduce whether the RLF was due to incorrect HO parameters, too early, too late, HO to wrong cell or due to a coverage hole, no cell with sufficient signal strength.
Radio Link Failure Indication
The purpose of the Radio Link Failure Indication procedure is to transfer information regarding RRC re-establishment attempts between eNBs controlling neighbouring cells. The signalling, as shown in FIG. 7, takes place from an eNB1 at which a re-establishment attempt is made to an eNB2 to which the user equipment concerned may have previously been attached prior to radio link failure.
The eNB2 initiates the procedure by sending an RLF INDICATION message to the eNB1 following a re-establishment attempt from a user equipment at the eNB2, when the eNB2 considers that the user equipment may have previously been served by a cell controlled by eNB1.
The RLF INDICATION message sent from eNB2 to eNB1 may comprise the following information elements:                A failure Cell ID: A PCI of the cell in which the user equipment was connected prior to the failure occurred.        A Reestablishment Cell ID: An E-CGI of the cell where RL re-establishment attempt is made.        A UE Identity: A C-RNTI and a MAC ID of the user equipment in the failure cell.        An RLF report: the eNB2 may include the UE RLF Report that it might have received via a UE Information Request, which may be used by the eNB1 to determine the nature of the failure.        
An RLF indication is used to communicate Too Late handovers.
Handover Report
If an eNB receives an RLF INDICATION message from a neighbour eNB, and if it finds out that it has sent a UE CONTEXT RELEASE message towards that neighbour eNB within the last Tstore_UE_cntxt seconds, this means that very recently the concerned user equipment was handed over properly to it from the same eNB, the eNB responds by sending a HANDOVER REPORT message that indicates Too Early Handover, as shown in FIG. 8.
If an eNB receives an RLF INDICATION message from a neighbour eNB, and if it finds out that it has sent a UE CONTEXT RELEASE message towards another neighbour eNB within the last Tstore_UE_cntxt seconds, this means that very recently the concerned user equipment was handed over properly to it from another eNB, the eNB responds by sending a HANDOVER REPORT message to the other eNB that indicates Handover to the Wrong Cell.
The HANDOVER REPORT message comprises:                A type of detected handover problem (Too Early Handover, Handover to Wrong        
Cell).                An E-CGI of source and target cells in the handover.        An E-CGI of the re-establishment cell, in the case of Handover to Wrong Cell.        A Handover cause, signalled by the source during handover preparation.        
Network Management
The management system assumed in this disclosure is shown in FIG. 9. The Node Elements (NE), also referred to as eNodeB, are managed by a Domain Manager (DM), also referred to as the operation and Operational Support System (OSS). A DM may further be managed by a Network Manager (NM) via a Ift-N interface. Two NEs are interfaced by X2, whereas the interface between two DMs is referred to as Itf-Point-to-Point (P2P). In this disclosure, it is further assumed that any function that automatically optimizes NE parameters may in principle execute in the NE, DM, or the NMs.
The mobility robustness optimization aims at avoiding handover failures including handover to wrong cell. However, when cells of various coverage areas are deployed, there might be situations where the connections are maintained, but the handover is still suboptimal. One such example is handover oscillations which may be counteracted based on UE history observations. Another example is short stay handovers where a user equipment quickly passes through a cell before being handed over to the next cell.